In positron emission tomography (PET) imaging, a radiopharmaceutical agent is administered, via injection, inhalation, and/or ingestion, to a patient. The physical and bio-molecular properties of the agent then concentrate at specific locations in the human body. The actual spatial distribution, intensity of the point and/or region of accumulation, as well as the kinetics of the process from administration and capture to eventual elimination, all have clinical significance. During this process, the positron emitter attached to the radiopharmaceutical agent emits positrons according to the physical properties of the isotope, such as half-life, branching ratio, etc.
Each positron interacts with an electron of the object, is annihilated and produces two gamma rays at 511 keV, which travel at substantially 180 degrees apart. The two gamma rays then cause a scintillation event at a scintillation crystal of the PET detector, which detects the gamma rays thereby. By detecting these two gamma rays, and drawing a line between their locations or “line-of-response,” the likely location of the original annihilation is determined. While this process only identifies one line of possible interaction, accumulating a large number of these lines, and through a tomographic reconstruction process, the original distribution is estimated with useful accuracy. In addition to the location of the two scintillation events, if accurate timing—within a few hundred picoseconds—is available, time-of-flight calculations are also made in order to add more information regarding the likely position of the annihilation event along the line. A specific characteristic of the isotope (for example, energy of the positron) contributes (via positron range and co-linearity of the two gamma rays) to the determination of the spatial resolution for a specific radiopharmaceutical agent.
The above process is repeated for a large number of annihilation events. While every case needs to be analyzed to determine how many scintillation events are required to support the desired imaging tasks, conventionally, a typical 100 cm long FDG (fluoro-deoxyglucose) study accumulates about 100 million counts or events.
Conventionally, as shown in FIG. 10, detection of an event 1000 is performed by a radiation detector, which includes a scintillator array 1002 and a photomultiplier tube (PMT) 1004. The event 1000 causes a scintillation event within the scintillator array 1002, producing light from an interaction of the energy from the event 1000 within a scintillator of the scintillator array 1002. The produced light is detected by the PMT 1004.
The PMT 1004 has an output signal which is filtered and output to a processing unit, which performs counting and time sampling of events, executing algorithms for determining energy, timing, and positions of the events. To increase the probability of the PMT 1004 detecting light produced via a scintillator, the scintillator array 1002 and the PMT 1004 are provided between reflective surfaces, reflectors 1006, which are designed to reflect the light produced via the scintillator.